Apparatus for gas plasma heating



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United States Patent 3,346,717 APPARATUS FOR GAS PLASMA HEATING Harold Pugh, Thelwall, Warrington, and George Gauterin,

Saughali, Chester, England, assignors to United Kingdom Atomic Energy Authority, London, England Original application Aug. 8, 1963, Ser. No. 300,774, new Patent No. 3,314,129, dated Apr. 18, 1967. Divided and this application Mar. 17, 1966, Ser. No. 554,233 Claims priority, application Great Britain, Aug. 14, 1962, 31,224/ 62 4 Claims. (Cl. 219-121) ABSTRAUT OF THE DISCLOSURE An evacuable container encloses a number of workpieces with a gas plasma generator and the workpieces are linked selectively with the generator by suitable circuitry. The workpieces are connected sequentially in one region after another as anodes with respect to the generator so that plasma from the generator is directed to various workpiece regions in turn.

This is a division of application Ser. No. 300,774, filed Aug. 8, 1966 now Patent No. 3,314,129, dated Apr. 18, 1967.

This invention relates to thermocouples and to apparatus applicable to the manufacture thereof.

One proposed method for the welding together of two thermocouple conductor wires makes use of a gas plasma derived from a plasma generator. The generator is mounted in one end of a vacuum vessel and the wires are mounted in the other end with abutting ends inside the vessel and are electrically connected as an anode with respect to the generator. The vessel is pumped down to a low pressure, the generator is energised and a voltage applied to the wires such that the abutting ends are subjected to a heating action by the gas plasma. In this way a fusion weld is effected at the abutting ends of the two wires.

The phrase gas plasma heating process is hereinafter used to denote the heating, in a rarefied atmosphere of a work piece by agas plasma derived from a plasma generator with respect to which the work piece is electrically connected as an anode, such a generator being understood to be one which, in the manner of a torch, ejects a gas plasma ionised internally such as by electrodes or by radiofrequency induction heating.

According to one aspect of the present invention a thermocouple cable having a tubular metal casing is subjected to a gas plasma heating process in which the casing forms the work piece so that one end of the casing is fused to form a sealed end closure.

This aspect of the invention has the particular advantage that it enables the casing to be end sealed without using a separate end cap.

According to a further aspect of the present invention a shaped end of a thermocouple cable having conductors surrounded by refractory electrical insulating material in a tubular metal casing is subjected to a gas plasma heating process in which first the conductors form the work 7 piece so that adjacent ends of the conductors are fused to form a thermocouple junction and second the casing forms the work piece so that the end of the casing is fused to form a sealed end closure insulated from the junction bythe insulating material.

The shaping of the end of the thermocouple cable preferably includes a substantially conical recess formed in the insulating material such that the conductors protrude into the bottom of the recess. p

This further aspect of the invention has particular advantage in that it enables the sealed end closure to be formed directly after the forming of the junction without disturbing the work piece, in other words there is no need to remove the cable from the rarefied atmosphere for treatment between the forming of the junction and the forming of the sealed end closure.

Thermocouples having a sealed end closure insulated from a junction in accordance with the invention have particular application for the measurement of high temperatures in reactive or corrosive environments. The sealed end closure insulated from the junction provides for the ready testing of the thermocouples and prevents leakage of corrosive or reactive material through the insulating material.

Also according to the invention, apparatus for carrying out a gas plasma heating process on a work piece assembly comprises an evacuable vessel to accommodate the assembly, a plasma generator adapted to provide a gas plasma in said vessel, means for scaling in passage through said vessel a plurality of electrical conductors to mutually insulated parts of the assembly, and an associated electrical circuit externally of said vessel, said circuit including selective switching means for sequentially connecting each of said assembly parts as an anode with respect to said plasma generator by way of said conductors.

By way of example, the invention will now be described in its application to the production of thermocouples from thermocouple cables each having Chromel and Alumel conductor wires embedded in compacted refractory electrical insulating material (e.g. magnesia) surrounded by a tubular stainless steel casing.

In the accompanying exemplifying drawings, FIGURES 1a to 1 show (on an enlarged scale) various stages in the production of a thermocouple from a thermocouple cable, FIGURE 2 shows diagrammatically the apparatus used, and FIGURE 3 is a schematic representation of the associated electrical circuit.

FIGURES 1a to 1 show a thermocouple cable 10 having conductor wires 11, 12 embedded in magnesia 13 surrounded by a tubular stainless steel casing 14.

The cable 10 is first cut to length and then lightly filed to give a flat end 15 (FIGURE 1a) of circular crosssection. The end is then shaped using a drill 16 (FIGURE lb) to include a conical recess 17 in the insulating material 13 of a depth of one cable diameter and then removing the insulating material between the wires to a further depth of one half of a cable diameter as shown by recess 18 (FIGURE 10) such that the Wires protrude into the bottom of the recess, care being taken not to disturb the insulating material projecting beyond the exposed ends of the wires. The exposed ends of the conductor wires are then bent toward each other as shown in FIGURE 1d. A length of close-fitting ceramic (e.g. alumina) tubing is then fitted over the cable casing so that the shaped end projects by one cable diameter. The other end of the cable is stripped of its casing and insulating material for a short length to expose the conductor Wires which are conveniently joined together for electrical connection purposes.

FIGURE 2 shows apparatus for carrying out a gas plasma heating process on a work piece assembly, the apparatus comprising a plasma generator comprising a tungsten cathode 20 sealed in an insulating housing 21, the cathode being associated with a water-cooled copper anode 22 having an orifice 23. The housing 21 is sealed to a brass end plate 24 in which the anode 22 is mounted. The plate 24 has a pipe connection 25 to a vacuum pump P and the housing has an inlet pipe 26 for argon gas from a cylinder C. The plate 24 is sealed to a cylindrical vacuum vessel 27 made of glass to the lower end of which is sealed a brass end plate 28. Mounted on the plate 27 in line with the orifice 23 is a stainless steel reflector rod 29 of circular section and having a concave surface 30.

One or more thermocouple cables with shaped ends prepared as described above may be mounted in the vessel. For convenience FIGURE 2 shows two such cables .31, 32 sealed by glands 32a in passage through the end plate 2 8, though it is to be understood that the apparatus is not limited to two thermocouple cable mounting points. In fact a typical arrangement comprises six cables equispaced round a 1.5 pitch circle diameter. The two cables 31, 32 are shown with their shaped ends inside the vessel projecting from ceramic tubing 33 and terminating in the general region of the reflecting zone 45 of the surface 30 of the reflector 29. Outside the vessel detachable electrical connections 34-, 35, 36, 37 are respectively made to casings 3'8, 39 and joined conductors 40, 41 of the cables 31, 32. These connections lead to a selector switch unit in turn connected to a bank of resistances 42 whose values are determined by the length and diameter of the cables mounted in the vessel. These resistances are connected to the positive side of a power pack 44 which has its negative side effectively connected to the anode 22. The anode 22 and cathode 20 are connected to a welding power pack with HF. initiator.

It will be appreciated that the electrical circuit associated with the apparatus of FIGURE 2 is provided with switching facilities to cater for the maximum number of thermocouple cables mounted in the vessel, and similarly that the circuit includes a range of components of selected values such as to cater for a range of cable sizes and lengths. Typically the apparatus caters for cable outer diameters in the range 0.01" (0.25 mm.) to /a" (3.1 mm.) and cable lengths of up to one hundred feet (30 m.).

Thus the electrical circuit of FIGURE 3 shows selective switching means suitable for the two thermocouple cables 31, 32 and electrical component values for cable dimensions of twenty foot (6 m.) length and 0.04" (1 mm.) outer diameter with a casing wall thickness of 0.004" (0.1 mm), each of the conductor wires then being 0.0065" (0.16 mm.) in diameter. The selector switch unit and cam switches 1 and 2 are operated by a cam shaft driven by an electric motor through a gear box, the motor circuit including push button and cam control to give the required operational sequence.

-In operation the vessel 27 is pumped down by the vacuum pump to a pressure of about 100 microns Hg and a short period of time allowed for outgassing of the shaped ends of the cables and the vessel. Argon is then supplied to the plasma generator by way of the pipe 26 until the pressure in the vessel rises to about mm. Hg (which pressure is maintained by continuous pumping) when the generator is energized using the Welding power pack with H. F. initiator to strike an are between the cathode 20 and anode 22 and thereby initiate a high temperature, highly ionized gas plasma 46 issuing into the vessel by way of the orifice 23.

The electrical circuit of FIGURE 3 is then energised to give the following operational sequence.

(1) The preheat timer is operated so that both casings 38, 39 are each connected through the relay and subjected to a preheat cycle (typically 30 seconds) by the plasma in which the two cables 31, 32 are heated up to 200 C. thereby drying out any moisture in their shaped ends.

The voltage of the power source (casings) is lower than that of the power source (conductors) and therefor results in a lower heating effect.

(2) The plasma conductivity is measure-d by push button application of a fixed low potential to a conductivity probe (which may be one of the cable sheaths) and measurement of the current. The conductivity can be adjusted by adjusting the pressure in the vessel. Typically for an applied potential of. volts DC, the current should be in the range of 50-60 ma.

(3) The motor circuit start button is pressed to give the following cycles.

(4) With the elector switch unit in the position shown, cam switch 1 is closed for a one second cycle during which the 500 rnfd. capacitor discharges through the joined conductors 41 of the cable 32. In this way, with the conductors of the cable 32 connected as an anode with respect to the plasma generator, they attract a concentration of the plasma and are therefore subjected to a gas plasma heating process so that the conductors are fused together at the shaped end of the cable 32 to form a thermocouple junction. Such a junction is indicated by reference numeral 47 in FIGURE 12.

(5) 0.5 second delay.

('6) Cam switch 2 is closed for a six second cycle during which the casing 39 is connected as an anode with respect to the generator by way of the 66 ohm resistance.

s As 6.

(10) As '6. In this way the casing 39 is subjected to a stepwise gas plasma heating process so that the casing at the shaped end of the cable 32 is fused to form a sealed end closure insulated from the junction by the insulating material. Such a sealed end closure is indicated by reference numeral 48 in FIGURE 1 (11) Contact is made to operate the selector switch unit so as to switch it to the lower position shown in FIG- URE 3, thus enabling cycles 4 to 10 above to be carried out for the cable 31.

The circuit is interlocked to ensure that the operator carries out the preheat cycle before initiating the fusion cycles. It will be appreciated that whilst the anode 22 is operatively maintained at a positive potential with respect to the cathode 20 (the Welding power pack with HF. initiator being designed to give volts open circuit and 20 volts at 10 amps under operating conditions), the work piece (as exemplified by the cables 31, 32) is operatively maintained at a positive potential with respect to the anode of the plasma generator so as to attract the plasma.

It is to be particularly noted that the apparatus described above provides a fully automatic sequence enabling each one of a number of shaped thermocouple cables to be sequentially operated on and thereby converted to a thermocouple having an integral sealed end closure insulated from its junction by the insulating material already present in the cable. This constitutes a significant achievement in the field of thermocouple development, as will be readily appreciated when it is pointed out that it was previously necessary to weld the conductors to form a junction, then add additional insulating material and a separate end cap which then had to be welded to the casing. By way of contrast, the present invention provides that once the shaped thermocouple cables have been mounted in the vessel 27, they do not then have to be removed until they are in their finished form complete with junction and sealed end closure, i.e. no separate end cap fitting is involved. This represents a considerable saving in time, as is evidenced by the above-described operational sequence from which it can be seen that the total time for steps 4 to 10 is less than half a minute, and that steps 1 to 10 can be carried out in less than a minute.

The conical recess formed in the shaped end of the thermocouple cable provides that the insulating material projecting upwardly beyond the junction is utilised to insulate the sealed end closure from the junction. It is noteworthy that the invention makes possible such fine concentration of the plasma as to be applicable to thermocouple cable of extremely small size. Thus, whilst the apparatus shown in FIGURES 2 and 3 of the drawings has been described with particular reference to its use with thermocouple cable of 0.04" (1 mm.) outer diameter, the invention is equally applicable to mineral insulated, metal sheathed, thermocouple cable having an outer diameter f y 0-01" -25 mm). Whilst not. essential. t P

sion of the reflector 29 with its surface 30 is advantageous in that the zone 45 defines a region of maximum electron density thus facilitating the fusion steps.

We claim:

1. Apparatus comprising a plasma generator for carrying out a gas plasma heating process on at least one workpiece assembly having mutually insulated parts, said apparatus comprising,

a sealed vessel to accommodate said assembly under vacuum,

a plurality of electrical conductors passing in a sealed manner into said vessel for establishing electrical connection with the mutually insulated parts of the assembly,

and an associated electric circuit externally of said vessel, said circuit including selective switching means for sequentially connecting each of said assembly parts as an anode with respect to said plasma generator by way of said conductors.

2. Apparatus according to claim 1 wherein said vessel includes reflector means comprising a reflector member mounted within the vessel in line with an orifice in said generator through which said plasma issues, the reflector member presenting a concave surface towards said orifice.

3. Apparatus according to claim 1 adapted to accommodate a plurality of workpiece assemblies wherein the switching means sequentially connects each workpiece assembly as an anode with respect to said plasma generator by way of said conductors.

4. Apparatus according to claim 1 including positioning means stationarily mounted within said vessel for positioning said workpiece assembly therein.

References Cited UNITED STATES PATENTS 1,552,349 9/1925 Ryan 314-36 2,390,596 12/1945 Larsen 219l59 RICHARD M. WOOD, Primary Examiner. J. V. TRUHE, Assistant Examiner. 

1. APPARATUS COMPRISING A PLASMA GENERATOR FOR CARRYING OUT A GAS PLASMA HEATING PROCESS ON AT LEAST ONE WORKPIECE ASSEMBLY HAVING MUTUALLY INSULATED PARTS, SAID APPARATUS COMPRISING, A SEALED VESSEL TO ACCOMMODATE SAID ASSEMBLY UNDER VACUUM, A PLURALITY OF ELECTRICAL CONDUCTORS PASSING IN A SEALED MANNER INTO SAID VESSEL FOR ESTABLISHING ELECTRICAL CONNECTION WITH THE MUTUALLY INSULATED PARTS OF THE ASSEMBLY, AND AN ASSOCIATED ELECTRIC CIRCUIT EXTERNALLY OF SAID VESSEL, SAID CIRCUIT INCLUDING SELECTIVE SWITCHING MEANS FOR SEQUENTIALLY CONNECTING EACH OF SAID ASSEMBLY PARTS AS AN ANODE WITH RESPECT TO SAID PLASMA GENERATOR BY WAY OF SAID CONDUCTORS. 